In many technical applications drive trains have to be used, which have to transfer a large mechanical power, wherein a conversion of input torque and rotation speed rate into appropriate output torque and rate is required. To this end, a wide variety of gear systems has been developed so as to comply with the various requirements in many technical fields. Generally, in a gear system at least two gears are engaged with each other so as to transfer mechanical forces in compliance with the application under consideration. The actual mechanical coupling between the various gear components takes place via the surface areas of the corresponding meshing components, such as the teeth of rotary or linear gear components. Consequently, corresponding compressive forces may act on the individual teeth flank, wherein the forces are transferred via the contact surfaces of the individual teeth meshing with each other. Therefore, the actually occurring mechanical pressure and thus the mechanical stress acting on the individual teeth significantly depends on the area or point or line of interaction, which in turn is affected by the overall profile of the teeth flanks. Generally, in high performance gearboxes the mechanical efficiency, the noise generation, the duration and the like are important aspects that have to be addressed in order to comply with the requirements of the various applications. For example, the engagement of the individual teeth of two gears can be adjusted so as to obtain an improvement in one or more of the above-mentioned aspects, for instance in view of noise reduction and the like, by appropriately adapting the flank profile of the teeth. To this end, frequently a so-called tooth trace correction is applied during the fabrication of one or more gears of a gearbox in order to appropriately adapt the tooth flanks to the expected load condition during operation of the gear system. For example, the leading flank of the teeth may be provided with a different profile compared to the trailing flank of the teeth, when substantially a predefined direction of rotation of the gears is encountered in the application under consideration. The process of tooth trace correction is thus a well-established concept for improving the load conditions for various components of the gearbox, for instance by applying appropriate additional manufacturing steps upon fabricating the individual gears of the gear system and/or by controlling the manufacturing process for the teeth, for instance the grinding process, in order to establish the teeth so as to have the desired target flank profile.
In many applications a substantially balanced load distribution in complex gearboxes is an essential aspect when the gearbox is designed for transferring high mechanical powers. For example, over the last decades wind energy has proven to be one important component for providing alternative energy due to the superior availability of wind energy and the moderately high cost-effectiveness of modern wind power plants. Presently, most of the highly efficient wind turbines are designed on the basis of wind rotors supported by a substantially horizontal shaft that in turn is mechanically coupled to a gearbox in order to convert the moderately low rotation speed of the wind rotor into a desired high rotation speed of an electrical generator. The conversion of the low input speed and high input torque into a high output speed and moderately low output torque is frequently accomplished on the basis of a planetary gear system, which thus has to transfer a mechanical power of several hundred kW to several MW, depending on the size of the wind power plant. Upon installing a wind power plant with horizontal rotor axis these gearboxes have to be mounted in the nacelle on a tower with a height of several 10 meters to 100 meters or more so that, for economical and technical reasons, generally a high power-to-weight ratio is desirable for the gearbox. For this reason, the weight of the gearbox is typically reduced as much as possible in order to obtain a desired power-to-weight ratio, thereby necessitating the components of the gearbox to be dimensioned closely to the material fatigue limits. On the other hand, the gearboxes have to be operated in remote locations, possibly in sophisticated environmental conditions, for instance offshore, so that regular maintenance intervals may represent an important cost factor, which in turn significantly influences the overall profitability of the wind power plant. Consequently, manufacturers of planetary gear systems for wind turbines have to meet very different requirements, for instance increasing the power-to-weight ratio, which requires reducing the amounts of required materials for the various gears, and providing superior durability at a reduced number of maintenance events over the lifetime of the wind turbine. The latter aspect may, however, require superior materials or an increased amount of material for enhancing the mechanical strength of the various components and/or the application of superior manufacturing techniques, such as sophisticated tooth trace corrections in order to reduce any load variations that may occur in the individual components of the planetary gear system.
Frequently, a gearbox structure is used in wind power plants in which a stationary ring gear of the planetary system engages with the planetary wheel whose carrier in turn is mechanically coupled to the shaft that supports the wind rotor. On the other hand, the sun gear is typically mechanically connected to an output shaft of the planetary gear system, which in turn may be coupled to a further gear system or to an electric machine. Consequently, the significant diameter of the wind rotor in combination with typically varying wind conditions, in particular in sophisticated environments, may result in a pronounced variation of the load conditions acting on the planetary gear system. That is, the torque transferred from the wind rotor via the shaft into the planetary stage of the gearbox may finally result in corresponding varying load conditions for the stationary ring gear and the sun gear. Consequently, great efforts are being made in estimating the load conditions for various operating states in order to apply sophisticated tooth trace corrections for the planetary wheels, the sun gear and the stationary ring gear. To this end, an appropriate tooth flank profile is determined for the teeth of at least one of these components in order to obtain reduced variability of the resulting load conditions. It turns out, however, that the conventional tooth trace correction concept may not efficiently address the varying load conditions, which are particularly introduced by the planetary wheel carrier connected to a mechanical load that induces strongly varying torque conditions, such as the wind rotor of a wind turbine.
In view of the situation described above, it is an object of the present invention to provide gearboxes and manufacturing techniques in order to provide for superior balance of load conditions in a gearbox.